The present invention relates to the control of torque or power from pneumatic tightening tools, and more specifically, to high speed pneumatic tools, such as impact and impulse tools, for purposes of tightening desired fasteners.
Impact and impulse tools are currently used extensively to tighten non-critical bolts in automotive and other industrial applications. Such tools provide very high torque to weight ratios, are very fast and have very low reaction torque since they effectively hammer the bolt tight. Unfortunately, however, the impacting action of the tools makes it difficult to control the tightening process since it is not possible to make accurate torque measurements, as it is with continuously operating tools. Consequently, such tools are rarely used in critical applications where bolts are required to be tightened precisely to a specified load or torque.
Techniques have been developed for performing direct load measurements in fasteners utilizing ultrasonic transducers which are removably, or preferably permanently attached to the fasteners. Examples of such techniques can be found, for example, in U.S. Pat. No. 6,990,866 (Kibblewhite); U.S. Pat. No. 6,009,380 (Vecchio et al.); U.S. Pat. No. 5,220,839 (Kibblewhite); U.S. Pat. No. 5,018,988 (Kibblewhite et al.); U.S. Pat. No. 4,899,591 (Kibblewhite); and U.S. Pat. No. 4,846,001 (Kibblewhite), each of which is incorporated by reference as if fully set forth herein. It has been found that such techniques make it possible to directly control the installation load of various different types of fasteners using all types of assembly tools, including impact and impulse tools.
Certain characteristics associated with impact and impulse tools, however, make them less desirable for use in critical applications. Firstly, if the tools are sized to tighten bolts quickly, to minimize assembly time, the angle of rotation per impact, and consequently the load increase per impact, can be large at the time that the specified load or torque is reached. Since the tools cannot be stopped during an impact, this results in significant tool overrun (i.e., final loads which exceed the specified loads), even when high speed solenoid valves are used to stop the tool.
Secondly, the rundown speed of such tools is extremely high, typically above 6,000 rpm. When these tools are used with prevailing torque lock nuts, locking fasteners or thread forming fasteners, rundown at these speeds can cause excessive localized heating in the threads of the fastener, resulting in undesirable changes in friction conditions or the degradation of friction coatings. This has been found to be common with the use of prevailing torque lock nuts in the aerospace industry, for example.